Hollow cathodes are the primary electron source in space propulsion applications, as well as in many ground-based devices such as gaseous lasers and plasma processing sources. They are often preferable to filament sources due to their increased robustness and lifetime. Hollow cathodes are cylindrical in shape, and consist of an orificed tube with a low work function material along the inner surface. See, e.g., Goebel, D. M., and Katz, I. (2008), Fundamentals of Electric Propulsion: Ion and Hall Thrusters, New York: Wiley; and Polk, J. et al. (2006, Jul. 9-12), “Characterization of Hollow Cathode Performance and Thermal Behavior,” AIAA-2006-5150, Sacramento, Calif. The ease with which the electrons are emitted from the insert is related to the work function of the material. See, e.g., Coulombe, S. and Meunier, J.-L (1997), “Thermo-field emission: a comparative study,” J. Phys. D: Appl. Phys., 30, 776-780; Murphy, E. L. and Good, R. H. (1956), “Thermionic Emission, Field Emission, and the Transition Region,” Physical Review, 102, 1464-1473; and Parlini, J. et al. (1993), “Thermo-field emission and the Nottingham effect,” Journal of Physics D: Applied Physics, 26, 1310. Lower work function indicates equivalent emission can be obtained at lower temperatures, improving the power efficiency because lower temperature cathodes lose less heat. A low temperature cathode has the potential to be extremely efficient and could be fabricated from inexpensive materials instead of refractory metals.
The calcium aluminate phase of 12CaO-7Al2O3 (C12A7), is one of several alumina-lime phases found in common alumina-based cements. C12A7 has a naturally formed nanostructure, in which subnanometer-sized cages form a three-dimensional crystal lattice. See, e.g., Y. Toda et al. (2007), “Work Function of a Room-Temperature, Stable Electride [Ca24Al28O64]4+(e−)4,” Advanced Materials, 19(21), 3564-3569. The unit cell consists of twelve cages. Although this cage structure is similar to those found in clathrate phases of ice and in zeolites, there is a difference in that the unit cell of C12A7 is positively charged; that is, there are four fewer electrons on the atoms that comprise the framework cage of C12A7 than are needed to neutralize the cage. The positive charge is counteracted by two atomic oxygen ions (O2−) that are clathrated (floating) within two of the twelve subcages. New properties can be imparted to C12A7 if the free oxygen ions are substituted with anions like O− and H−, and when four electrons are substituted for the two O2− ions to form C12A7 electride, the only inorganic electride known to be stable at high temperature. See e.g., S. Matsuishi et al. (2003), “High-Density Electron Anions in a Nanoporous Single Crystal: [Ca24Al28O64]4+(4e−). Science, 301, 626-629; and S. Kim et al. (2007),” Fabrication of room temperature-stable 12CaO 7Al2O3 electride: a review,” Journal of Material Science, 18, S5-S14. The stability of the C12A7 electride is attributable to the unique cage structure as well as the fully oxidized nature of the lattice.
The work functions of current state-of-the-art hollow cathode insert materials lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) are near 2.7 eV, while the work function of barium-impregnated porous tungsten (Ba—W) is near 2.1 eV (D. Goebel et al. (2007), “LaB6 Hollow Cathodes for Ion and Hall Thrusters,” Journal of Propulsion and Power,” 23(3), 552-558. LaB6 and CeB6 are generally heated to approximately 1900 K to obtain sufficient levels of emission, while Ba—W is heated above 1300 K. See e.g., D. Goebel et al., supra. These temperatures require well-made heaters and good thermal insulation. Ba—W cathodes, while operating at lower temperatures, are more susceptible to both poisoning and high rates of evaporation if operated at high current See. e.g., D. Goebel et al., supra. By contrast, the work function of C12A7 electride has been measured in field emission tests to be as low as 0.6 eV, due to its unique charged lattice structure. See, e.g., S. Kim et al. (2006), “Synthesis of a Room Temperature Stable 12CaO.7Al2O3 Electride from the Melt and Its Application as an Electron Field Emitter,” Chem. Mater., 18(7), 1938-1944; and J. E. Medvedeva et al. (2007), “Electronic band structure and carrier effective mass in calcium aluminates, Physical Review B, 76, 155107-1-155107-6; and Y. Toda et al. (2004), “Field Emission of Electron Anions Clathrated in Subnanometer-Sized Cages in [Ca24Al28O64]4+(4e−),” Advanced Materials, 16(8), 685-689.